Many known DC devices such as electromagnets and energy storage cells have, for certain applications, been replaced by equivalent superconducting devices whose performance generally is far superior. In contrast to the DC case, however, when AC magnetic fields and/or currents are involved, superconducting devices appear to possess energy losses which may be comparable to or even greater than their conventional counterparts. Although the energy losses in superconductors depend upon both frequency and magnetic reversibility, it is presently believed that the major limitation in the performance of AC superconducting devices is due to the presence of hysteretic losses arising from the magnetic irreversibility in the superconductor. For this reason, superconducting AC devices depend for their practicality upon reducing the energy (AC) losses to an acceptably low magnitude. The energy loss in a superconducting AC device is determined by the following factors:
1. OPERATING FREQUENCY;
2. PEAK AMPLITUDE OF THE AC magnetic field at the surface of the superconductor;
3. OPERATING TEMPERATURE;
4. PHYSICAL PROPERTIES OF THE SUPERCONDUCTING MATERIAL EMPLOYED; AND
5. THE GEOMETRICAL AND PHYSICAL CHARACTERISTICS OF THE SURFACE.
In most cases the operating frequency is determined by the nature of the device and is therefore essentially fixed. The second and third factors influence not only the energy loss but the performance of the superconducting device as well. In AC devices such as RF cavities for beam separators the magnetic field is required to be as high as is possible for the superconductors employed. In addition, since the critical field of superconductors increases with decreasing temperature it is necessary to operate such devices at as low a temperature as is economically practical. Hence, the only remaining factors which may be truly varied to minimize the energy losses are the physical properties and surface characteristics of the superconductor employed. An ideal material would exhibit zero loss at the highest magnetic field. Of all elemental superconductors, niobium is preferred for AC applications where a high magnetic field is required. Niobium in its commercially available state, however, will yield relatively high AC losses. Recent experimentation has established that the presence of impurities in the virgin material is the major contributing factor to such AC losses. Thus, ultra-high purity niobium should and does exhibit almost perfect magnetic reversbility and as such almost zero electrical resistivity. Equally important is the reduction of AC losses attributable to surface imperfections which would otherwise limit the advantages to be gained from the use of ultra-high pure niobium in a practical superconducting device. This further complicates the fabrication of a superconducting AC device where complex shapes are involved. In fact, many uncomplicated superconductor geometries, depending upon the desired location for the active superconductor surface, are not susceptible to presently known techniques for removing surface irregularities.
The fabrication of a superconducting radio frequency (RF) cavity highlights the difficulties presently encountered in the art relating to the general fabrication of superconducting AC devices where low energy losses are desirable. In principle, all that is required for a superconducting niobium RF cavity is a solid high purity niobium structure having the desired shape and surface finish. As a practical matter, however, to machine an RF cavity from a solid piece of niobium where complex internal shapes are involved is nearly impossible not to mention the prohibitive cost of the high purity niobium required. Furthermore, except for the very highest purity niobium the losses are such that the thermal conductivity of thick niobium walls would be too low to allow the heat dissipated in the cavity to be extracted at the rate necessary to maintain the inner wall of the cavity at a temperature sufficiently close to that of the surrounding liquid helium bath employed to maintain the material in the superconducting state. At present, therefore, no satisfactory technique exists for fabricating an AC superconducting article, such as an RF cavity, in one piece, where low AC losses are required.